Microelectronic mechanical system and methods

ABSTRACT

The current invention provides for encapsulated release structures, intermediates thereof and methods for their fabrication. The multi-layer structure has a capping layer, that preferably comprises silicon oxide and/or silicon nitride, and which is formed over an etch resistant substrate. A patterned device layer, preferably comprising silicon nitride, is embedded in a sacrificial material, preferably comprising polysilicon, and is disposed between the etch resistant substrate and the capping layer. Access trenches or holes are formed in to capping layer and the sacrificial material are selectively etched through the access trenches, such that portions of the device layer are release from sacrificial material. The etchant preferably comprises a noble gas fluoride NGF 2x  (wherein Ng=Xe, Kr or Ar: and where x=1, 2 or 3). After etching that sacrificial material, the access trenches are sealed to encapsulate released portions the device layer between the etch resistant substrate and the capping layer. The current invention is particularly useful for fabricating MEMS devices, multiple cavity devices and devices with multiple release features.

REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.10/268,257, filed on Oct. 9, 2002, now U.S. Pat. No. 7,049,164 which isa divisional of U.S. application Ser. No. 09/952,626, filed on Sep. 13,2001 now U.S. Pat No. 6,930,364.

FIELD OF THE INVENTION

The present invention relates to wafer processing. More particularly,the present invention relates to methods for encapsulation ofmicroelectronic mechanical systems.

BACKGROUND OF INVENTION

The combination microelectronic mechanical systems (MEMS) and integratedcircuits (ICs) allows for the possibility to make any number ofmicro-sensors, transducers and actuators. Unfortunately, typical methodsfor making MEMS are incompatible methods used to fabricate ICs. Hence,MEMS and ICs are usually fabricated separately and laboriously combinedin subsequent and separate steps.

In addition to the MEMS and ICs processing incompatibilities, MEMStypically require encapsulation, whereby the active portions of the MEMSare sealed within a controlled storage environment. One way toencapsulate the active portions of the MEMS is to provide uniquecustomized packaging structure configured with conductive leads fittedfor the MEMS. Alternatively, the MEMS can be formed on a wafer substratethat serves as a bottom portion of the packaging structure. After theMEMS is formed on the wafer, then a matched lid structure is glued orsoldered over the active potions of the MEMS within the suitable storageenvironment. For example, Shook describes a method and apparatus forhermetically passivating a MEMS on a semi-conductor substrate in U.S.patent application Ser. No. 09/124,710, and also U.S. patent applicationSer. No. 08/744,372, filed Jul. 29, 1998 and entitled METHOD OF ANDAPPARATUS FOR SEALING A HERMETIC LID TO A SEMICONDUCTOR DIE, thecontents of both of which are hereby incorporated reference.

What is needed is a method to make MEMS and other structures on a wafersubstrates utilizing processes that are compatible with standard ICwafer processing, whereby MEMS and ICs are capable of being fabricatedon the same wafer chip. Further, what is needed is a method to fabricateMEMS, wherein the active portions of the MEMS are readily encapsulatedwithin a variety of suitable storage environments.

SUMMARY OF THE INVENTION

The current invention provides a method of making an encapsulatedrelease structure. Preferably, the release structure is a MEMS devicehaving a plurality of ribbons or beams, which may further have a combstructure. In an embodiment of the instant invention, the devicecomprises a resonator that can be used for periodic waveform generation(e.g. clock generation). In other embodiments, the device comprises agrating light valve for generation and/or transmission of opticalinformation. In yet other embodiments the device comprises a radiofrequency (RF) generator for wireless transmission of information.

The release structure is formed between layers of a multi-layerstructure. The multi-layer structure preferably comprises a first andsecond etch-stop layers, which can be the same as or different from eachother, and a first sacrificial layer between the first and the secondetch-stop layer. Release features are patterned into the secondetch-stop layer. Preferably, the multi-layer structure is formed on asilicon wafer substrate. The silicon wafer substrate is preferablyconfigured to couple the MEMS device with an integrated circuit (IC),also formed on the silicon wafer substrate.

Preferably, the multi-layer structure is formed with a first etch-stoplayer that is deposited on or over a selected region of the siliconwafer substrate. The first etch-stop layer is preferably a silicondioxide layer, a silicon nitride layer or a combination thereof. On topof or over the first etch-stop layer the first sacrificial layer isformed. The first sacrificial layer preferably comprises a polysiliconmaterial though other materials can also be used. The second etch-stoplayer is formed on or over the first sacrificial layer with a patterncorresponding to release features of the release structure.

The second etch-stop layer is patterned with the release structurefeatures using any suitable patterning technique. Accordingly, apatterned photo-resist is formed on or over the second etch-stop layerprior to removing a portion thereof to form a patterned second etch-stoplayer having gaps therein and between portions of the second etch-stoplayer under the patterned phot resist. Alternatively, the firstsacrificial layer can be anisotropically etched with a positiveimpression of the release structure features. The positive impression ofthe release structure features provides nuclei for rapid anisotropicgrowth of release structure features onto the patterned portions of thefirst sacrificial layer during the deposition of the second etch-stoplayer. Regardless, of the method used to form the second etch-stoplayer, a second sacrificial layer is formed over the second etch-stoplayer sandwiching the second etch-stop layer having the releasestructure features between the first and the second sacrificial layers.The second sacrificial layer preferably comprises polysilicon. On top ofthe second sacrificial layer a sealant layer or capping layer is formed.The capping layer preferably comprises one or more conventionalpassivation layers and more preferably comprises a silicon oxide layer,a silicon nitride layer or a combination thereof.

The etch-stop layers are formed by any number of methods. An etch-stoplayer can be formed from any materials that show resistance to etchingunder specified etching conditions relative to the materials that formthe sacrificial layer(s). In the instant invention the etching rate(mass or thickness of material etched per unit time) of sacrificialmaterials(s) relative to the etch-stop layer materials is preferablygreater than 10:1, more preferably greater than 50:1 and most preferablygreater than 100:1. In developing the present invention, experimentalresults of approximately 2500:1 have been achieved. Any particularetch-stop layer can comprise one or more layers, any of which can beexposed to the sacrificial layer etchant as long as the etch-stop layerexhibits sufficient resistance to the sacrificial layer etchant.

In an embodiment of the instant invention, one or more of the etch-stoplayers of the multi-layer structure comprise silicon oxide. Preferablythe silicon oxide is silicon dioxide; when silicon oxide is referred toin this document, silicon dioxide is the most preferred embodiment,although conventional, doped and/or non-stoichiometric silicon oxidesare also contemplated. Silicon oxide layers can be formed by thermalgrowth, whereby heating a silicon surface in the presence of an oxygensource forms the silicon oxide layer. Alternatively, the silicon oxidelayers can be formed by chemical vapor deposition processes, whereby anorganic silicon vapor source is decomposed in the presence of oxygen.Likewise, the silicon nitride layers can be formed by thermal growth orchemical deposition processes. The polysilicon sacrificial layers arepreferably formed by standard IC processing methods, such as chemicalvapor deposition, sputtering or plasma enhanced chemical vapordeposition (PECVD). At any time before the formation of a subsequentlayer, the deposition surface can be cleaned or treated. After the stepof patterning the release structure, for example, the deposition surfacecan be treated or cleaned with a solvent such as N-methyl-2-pyrolipone(NMP) in order to remove residual photo-resist polymer. Further, at anytime before the formation of a subsequent layer, the deposition surfacecan be mechanically planarized.

After the multi-layer structure is formed with the release structure(e.g. patterned from the second etch-stop) sandwiched between the firstand the second sacrificial layers, access holes or trenches are formedin the capping or sealant layer, thereby exposing regions of the secondsacrificial layer therebelow. Access trenches are referred to, herein,generally as cavitations formed in the capping or sealant layer which isallows the etchant to etch the material in the sacrificial layertherebelow. For simplicity, the term access trenches is used herein toencompass both elongated and symmetrical (e.g. holes, rectangles,squares, ovals, etc.) cavitations in the capping or sealant layer.

In accordance with the instant invention, access trenches can have anynumber of shapes or geometries, but are preferably anisotropicallyetched to have steep wall profiles. The access trenches are preferablyformed by etching techniques including wet etching processes andreactive ion etching processes though other conventional techniques canbe used. The exposed regions of the second sacrificial layer are thentreated to a suitable etchant which selectively etches substantialportions of the first and second sacrificial layers portion so therelease structures are suspended under the capping or sealant layer.

The preferred etchant comprises a noble gas fluoride, such as xenondifluoride. Preferably, the exposed regions of the second sacrificiallayer can be treated with a pre-etch solution of ethylene glycol andammonium fluoride prior to selectively etching the first and secondsacrificial layers. The pre-etch solution can prevent the formation ofoxide, clean exposed regions of the second sacrificial layer, removepolymers and/or help to ensure that etching is not quenched by theformation of oxides. The etching step is preferably performed in achamber, wherein the etchant is a gas. However, suitable liquid etchantsare considered to be within the scope of the current invention, wherebythe noble gas fluoride is a liquid or is dissolved in suitable solvent.

In the preferred method of the instant invention the multi-layerstructure is placed under vacuum with a pressure of approximately 10⁻⁵Torr. A container with Xenon Difluoride crystals is coupled to thechamber through a pressure controller (e.g. a controllable valve). Thecrystals are preferably at room temperature within the container withthe pressure of Xenon Difluoride of approximately 4.0 Torr. The pressurecontroller is adjusted such that the pressure within the chamber israised to approximately 50 milliTorr. This pressure, or an alternativelysufficient pressure, is provided to ensure a controllable etching rate,a positive flow of Xenon Difluoride to the chamber and excellentuniformity of the etch processes.

After the etching step, the access trenches maybe sealed to encapsulatethe suspended release structure between the first etch-stop layer andthe capping or sealant layer. The sealing step is performed at aseparate processing station within a multi-station wafer processingsystem or, alternatively, is performed within the chamber apparatus. Theaccess trenches can be sealed by any number of methods includingsputtering, chemical vapor deposition (CVD), plasma enhanced chemicalvapor deposition (PECVD), or spin on glass methods. The access trenchescan be sealed with any number of materials including metals, polymersand ceramics. Preferably, the access trenches are sealed by sputtering alayer of aluminum over the access trenches and the capping layer. Foroptical applications, excess aluminum can be removed from the capping orsealant layer using a suitable mechanical or chemical method.

In accordance with alternative embodiments of the invention, beforedepositing the second sacrificial layer on the patterned secondetch-stop layer, the second etch-stop layer may have a reflectivematerial deposited thereon. The reflective material preferably comprisesaluminum. Accordingly, after the sacrificial layers are etched away, therelease features preferably have a reflective upper surface suitable foroptical applications.

In yet other embodiments of the invention, a gettering material, such astitanium or a titanium-based alloy can be deposited within a cavitycapped by the capping or sealant layer prior to sealing the accesstrenches in the capping or sealant layer. The gettering material isprovided to help reduce residual moisture and/or oxygen which can leadto performance degradation of the device over time. The releasestructure is preferably sealed under a vacuum or, alternatively, under asuitable noble gas atmosphere, as described in detail below.

The invention provides a sealed MEMS device on an IC chip, intermediateelements thereof and also a method of forming the same using techniquesthat are preferably compatible with standard IC processing. For example,the method of the instant invention provides for processing steps thatare preferably carried out at temperatures below 600 degrees Celsius andmore preferably at temperatures below 550 degrees Celsius. Further, thecurrent invention provides for a method to fabricate MEMS with activestructures which are hermetically sealed in a variety of environments.The current invention is not limited to making MEMS and can be used tomake any number of simple or complex multi-cavity structures that havemicro-fluid applications or any other application where an internalizedmulti-cavity silicon-based structure is preferred. Also, as will beclear for the ensuing discussion that the method of the instantinvention is capable of being used to form any number of separate orcoupled release structures within a single etching process and thatlarger devices can be formed using the methods of the instant invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustrating a MEMS oscillator.

FIGS. 2 a–h illustrate top views and cross-sectional views a multi-layerstructure formed on silicon wafer substrate, in accordance with currentinvention.

FIGS. 3 a–f show cross sectional views of a release features beingformed from a multi-layer structure, in accordance with a preferredmethod of the current invention.

FIG. 4 is a block diagram outlining steps for forming a multi-layerstructure illustrated in FIG. 3 a.

FIG. 5 is a block-diagram outlining the method of forming a releasestructure from the multi-layered structure shown in FIG. 2 a.

FIG. 6 is a block-diagram outlining the steps for etching sacrificiallayers of the multi-layer structure illustrated in FIG. 2 b.

FIG. 7 is a schematic diagram of a chamber apparatus configured to etcha multi-layered structure formed in accordance with the method ofinstant invention.

DETAILED DESCRIPTION OF THE INVENTION

In general, the present invention provides a method to make devices withencapsulated release structures. The current invention is particularlyuseful for fabricating MEMS oscillators, optical display devices,optical transmission devices, RF devices and related devices. MEMSoscillators can have any number or simple or complex configurations, butthey all operate on the basic principle of using the fundamentaloscillation frequency of the structure to provide a timing signal to acoupled circuit. Referring to FIG. 1, a resonator structure 102 has aset of movable comb features 101 and 101′ that vibrate between a set ofmatched transducer combs 105 and 105′. The resonator structure 102, likea pendulum, has a fundamental resonance frequency. The comb features 101and 101′ are secured to a ground plate 109 through anchor features 103and 103′. In operation, a dc-bias is applied between the resonator 102and a ground plate 109. An ac-excitation frequency is applied to thecomb transducers 105 and 105′ causing the movable comb features 101 and101′ to vibrate and generate a motional output current. The motionaloutput current is amplified by the current to-voltage amplifier 107 andfed back to the resonator structure 102. This positive feed-back loopdestabilizes the oscillator 100 and leads to sustained oscillations ofthe resonator structure 102. A second motional output current isgenerated to the connection 108, which is coupled to a circuit forreceiving a timing signal generated by the oscillator 100.

Referring now to FIG. 2 a showing a plan view of a wafer, a waferstructure 200 preferably comprises a silicon substrate 201 and a firstetch-stop layer 203. The first etch-stop layer 203 may not be requiredto perform the methods of the instant invention, especially when thesilicon substrate 201 is sufficiently thick to allow sacrificial layersto be etched without completely etching away the silicon substrate 201.Also, the substrate 201 itself can be formed from or doped with amaterial that renders the substrate 201 substantially resistant to theetchant that is used, such that the formation of the first-etch-stoplayer 203 is not required. However, in an alternative embodiment, amaterial that can be selectively etched relative to a silicon substratecan be selected or used as the sacrificial layer. The first etch-stoplayer 203 preferably comprises silicon oxide, silicon nitride, acombination thereof or any other suitable material which exhibitssufficient resistance to the etchant used to etch the first sacrificiallayer.

Still referring to FIG. 2 a, a region 251 of the wafer structure 200 isused to form the release structure. Other portions of the waferstructure 200 can be reserved for forming an integrated circuit that canbe electrically coupled to and that can control operation of the releasestructure formed in the region 251. In addition, any number of releasestructures and release structure region 251 can be formed on the samewafer structure 200.

Now referring to FIG. 2 b, in the region 251, a first sacrificial layer205 is formed over the first etch-stop layer 203 using any conventionaltechnique. The first sacrificial layer 205 is formed from any suitablematerial that is selectively etched relative to the underlying firstetch-stop layer(s), but preferably comprises polysilicon.

Referring now to FIG. 2 c, a second etch-stop layer 207 is formed overthe first sacrificial layer 205. The second etch-stop layer 207 can beformed of the same or different material as the first etch-stop layer203. The second etch-stop layer 207 preferably comprises silicon oxide,a silicon nitride, a combination thereof or any other suitable materialwhich exhibits sufficient resistance to the etchant used. In anembodiment of the invention, the first sacrificial layer 205 is etchedprior to depositing the second etch-stop layer 207 to provide raisedsupport features 215 and 215′ which support the subsequently formedrelease structures. Alternatively, or in addition to forming the raisedsupport features 215 and 215′, support posts may be formed 216, 216′ and216″ in positions to provide support for the release structures formedin subsequent steps. Preferably, the support posts 216, 216′ and 216″are formed from an etch resistant material(s) that are the same ordifferent than material(s) used to form the etch-stop layer 203 and/oretch-stop layer 207 and capping layer 211, as described in detail below.

Alternatively to forming support features 215 and 215′ and/or supportposts 216, 216′ and 216″, or in addition to forming the support features215 and 215′ and/or support posts 216, 216′ and 216″, the secondetch-stop layer 207 can be deposited in an area of the region 251without underlying sacrificial layer 205 and such portions of the secondetch-stop layer 207 maybe deposited directly onto and/or attached to thefirst etch-stop layer 203 and/or substrate 201, such as shown in FIG. 2d. After the second etch-stop layer 207 is patterned and the sacrificiallayer 205 is etched, portions of the second etch-stop layer 207deposited directly on the first etch-stop layer 203 provide structuralsupports for the release structures formed. There are any number ofmechanisms to provide physical support for the release structures formedthat are considered to be within the scope of the instant invention.

Now referring to FIG. 2 e, in accordance with a preferred embodiment ofthe instant invention a reflective layer 233 is deposited over thesecond etch-stop layer 207 and/or the support features 215 and 215′and/or support posts 216, 216′ and 216″. The reflective layer 233preferably comprises aluminum or other suitable reflective material. Thereflective layer 233 is preferably resistant to enchant being used inremoving the sacrificial layers, but is capable of being etched usingother suitable techniques including photo-lithograph and plasma etch,wherein the patterned release structures formed in subsequent steps havereflective surfaces suitable for optical applications. Preferably, a setof bond pad 226, 227 and 228 are also formed on the wafer structure 200for electrically coupling the release structure(s) to a circuit externalto the integrated circuit containing/comprising the releasestructure(s). It will be readily understood by those of ordinary skillin the art that the reflective layer 233 can alternatively be depositedon the release features 204 and 206 after they are formed.

Now referring to FIG. 2 f, the reflective layer 233 and the secondetch-stop layer 207 is patterned to form the release structures/features204 and 206. The reflective layer 233 and the second etch-stop layer 207are preferably patterned using conventional photo-lithography techniquesand/or steps. For example, a photo-resist layer is formed on thereflective layer 233. The photo-resist is patterned and developed toform a patterned phot-resist mask (not shown). Portions of thereflective layer 233 and the second etch-stop layer 207 are then removedusing conventional techniques leaving the patterned features 204 and 206with a reflective layer 233 under the patterned photo-resist mask. Thepatterned photo-resist mask can then be removed from the patternedfeatures 204 and 206 and the patterned features 204 and 206 can beencapsulated as described in detail below.

Alternatively, the first sacrificial layer 205 can be etched with apositive impression of the release features (not shown). The positiveimpression of the release features then provide nuclei for rapidanisotropic growth of release structure features 204 and 206. Therelease features 204 and 206 are shown in FIG. 2 f as comb structures.However, it is clear that the release features can be comb structures,ribbon structures, cantilevers or any number of other structuresincluding, but not limited to, domain separators, support structuresand/or cavity walls as described in detail below. Further, whileproviding a reflective layer 233 is preferred, the additional step offorming a reflective layer 233 is not required when the patternedfeatures 204 and 206 are not used to reflect light, such as in the casefor micro-fluidic devices. The line 270 shows an x-axis of the waferstructure 200 and the line 271 shows the y-axis of the wafer structure.The z-axis 272 of the wafer structure 272 in FIG. 2 f is normal to theview shown.

FIG. 2 g shows a side cross-sectional view of the wafer structure 200after a second sacrificial layer 209 is deposited over release features204 and 206 with the reflective layer 233. In the FIG. 2 g, the y-axis271 is now normal to the view shown and the z-axis 272 in now in theplane of the view shown. The release features 204 and 206 are embeddedbetween the sacrificial layers 205 and 209 and the sacrificial layers205 and 209 are preferably in contact through gap regions between therelease features 204 and 206. The second sacrificial layer 209 is formedof any suitable material that is selectively etched relative to theetch-stop layer(s) used to form the release structure device, butpreferably comprises polysilicon.

Now referring to FIG. 2 h, after the second sacrificial layer 209 isdeposited over the release features 204 and 206, a capping layer 211 isdeposited over the second sacrificial layer 209. The capping layer 211preferably comprises silicon dioxide, silicon nitride any combinationthereof or any other suitable material(s) which exhibit(s) sufficientresistance to the etchant used. The capping layer 211 can be formed ofthe same or different material as the first etch-stop layer 203 and/orthe second etch-stop layer 207. FIGS. 3 a–3 f will now be used toillustrate the preferred method of forming an encapsulated releasestructure from a portion 250 of the structure 200 as shown in FIG. 2 h.

Referring now to FIG. 3 a, a device with a release structure, such asthe MEMS resonators structure 102 described above, is preferably madefrom a multi-layer structure 250. The multi-layer structure 250 has afirst etch-stop layer 203 that is preferably formed on the region 251 ofthe silicon wafer substrate 201, such as previously described. The firstetch-stop layer 203 may comprise any material or materials that exhibitresistance to etching under the conditions for etching the firstsacrificial layer. For example, when the first etch sacrificial layercomprises polysilicon, the first sacrificial layer etchant comprisesXeF₂, and the first sacrificial layer etching conditions are describedbelow for etching polysilicon with XeF₂. The first etch-stop layer 203preferably comprises a silicon oxide layer or a silicon nitride layerwith a layer thickness in a range of 500 to 5000 Angstroms.

On top of the first etch-stop layer 203 there is formed a firstsacrificial layer 205. The first sacrificial layer 205 may comprise anymaterials(s) that may be selectively etched relative to the underlyingfirst etch-stop layer 203 (when present) or substrate 201 (when thefirst etch-stop layer is not present). However, when the first etch-stoplayer 203 comprises silicon oxide or silicon nitride, the firstsacrificial layer 205 preferably comprises a polysilicon. Alternatively,the first sacrificial layer 205 can comprise a doped silicon oxide layerthat is doped with boron, phosphorus or any other dopant which rendersthe first sacrificial layer 205 to be preferentially etched over thesubstrate 201 or etch-stop layer 203 and/or the etch-stop layer 206 andcapping layer 211, described in detail below. The first sacrificiallayer 205 preferably has a layer thickness in a range of 0.1 to 3.0microns.

On top of the first sacrificial layer 205 is formed a second etch-stoplayer 207. The second etch-stop layer 207 is patterned with features 206and 204 corresponding to the release structure. The first etch-stoplayer 203 may comprise any material(s) that exhibit resistance toetching under the conditions for etching the first sacrificial layer.For example, when the first sacrificial layer 205 comprises polysilicon,the first sacrificial layer etchant comprises XeF₂, and the firstsacrificial layer etching conditions are described below for etchingpolysilicon with XeF₂. The second etch-stop layer 207 preferablycomprises a silicon oxide layer or a silicon nitride layer with a layerthickness in a range of 300 to 5000 Angstroms.

On the second etch-stop layer 207 is formed a second sacrificial layer209. The second sacrificial layer 209 may comprise any materials(s) thatmay be selectively etched relative to the underlying, the secondetch-stop layer 207 and/or the first etch stop layer 203 (when present)or substrate (when the first etch-stop layer is not present). However,when the first and the second etch-stop layers 203 and 207 comprisesilicon oxide or silicon nitride, the second sacrificial 209 layerpreferably comprises a polysilicon. Alternatively, second firstsacrificial layer 209 can comprise a doped silicon oxide layer that isdoped with boron, phosphorus or any other dopant which renders thesacrificial layer 209 to be preferentially etched over the substrate 201or etch-stop layers 203 and 207. The second sacrificial layer 209preferably has a layer thickness in a range of 0.1 to 3.0 microns andpreferably, the sacrificial layers 205 and 209 are in contact with eachother in the patterned regions 208 or gaps between the features 206 and204 of the release structure.

A capping or sealant layer 211 is deposited over second sacrificiallayer 209. The capping or sealant layer 211 preferably comprises aconventional passivation material (e.g. an oxide, nitride, and/or anoxynitride of silicon, aluminum and/or titanium). The capping or sealantlayer 211 also can comprise a silicon or aluminum-based passivationlayer which is doped with a conventional dopant such as boron and/orphosphorus. More preferably, the capping layer or sealant layer 211comprises a silicon oxide layer with a layer thickness in a range of 1.0to 3.0 microns. It will be apparent to one of ordinary skill in the artthat though the layers referred to above are preferably recited as beingsingle layer structures, each can be formed of a sandwich of knownlayers to achieve the same result. Furthermore, though the layers arepreferably taught as being formed one on top of the next, it will beapparent that intervening layers of varying thicknesses can be inserted.

Now referring to FIG. 3 b, access trenches 213 and 219 are formed in thecapping layer 211 thereby exposing regions 215 and 217 of the secondsacrificial layer 209. The access trenches 213 and 219 are preferablyanisotropically etched, although the access trenches 213 and 219 may beformed by any number of methods including wet and/or dry etchingprocesses. For example, a photo-resist is provided on the capping layerand is exposed and developed to provide a pattern for anisotropicallyetching the access trenches 213 and 219. Alternatively, an etchant maybe selectively applied to a portion of the etch-stop layer 211corresponding to the access trenches 213 and 219. For examplemicro-droplets or thin streams of a suitable etchant can be controllablyapplied to the surface of the capping or sealant layer 211 using amicro-syringe technique, such as described by Dongsung Hong, in U.S.Patent Application No. 60/141,444, filed Jun. 29, 1999 (Attorney DocketNo. 0325,00226), the contents of which are hereby incorporated byreference.

After the access trenches 213 and 219 are formed in the capping layer211, when the second sacrificial layer comprises polysilicon, theexposed regions 215 and 217 of the second sacrificial layer 209 can betreated with a pre-etch solution of ethylene glycol and ammoniumfluoride. A suitable pre-mixed solution of ethylene glycol and ammoniumfluoride is commercially available under the name of NOE Etch I™manufactured by ACSI, Inc., Milpitas, Calif. 95035. Oxides can form onthe surfaces of exposed polysilicon regions, such as 215 and 217. Suchoxides can interfere with polysilicon etching and result in anincomplete etch. The pre-etch solution is believed to prevent and/orinhibit the formation of oxides on the surfaces of the exposed regions215 and 217, or removes such oxides if present and/or formed, to avoidincomplete etching of the sacrificial layers 205 and 209.

Now referring to FIG. 3 c, after the access trenches 213 and 219 areformed in the capping layer 211, the sacrificial layers 205 and 209 areselectively etched to release the features 204 and 206. The features 204and 206 can have any number of different geometries. For example, in thefabrication of a MEMS device the release features are comb or ribbonstructures. In the fabrication of a micro-fluidic device the releasefeatures provide pathways which interconnect cavities 221 and 223. Inthe fabrication of electronic levels or electronic accelerometers therelease features can be cantilevers. After the features 204 and 206 arereleased, then the access trenches 213 and 219 in the layer 211′ aresealed to encapsulate the features 204 and 206 between the layers 203and 211′.

Now referring to FIG. 3 d, in further embodiments of the instantinvention, prior to sealing the access trenches 213 and 219 in the layer211′, a gettering material 231 such as titanium or a titanium-basedalloy can be deposited within at least one of structure cavities 221 and223 through the access trenches 213 and 219. Alternatively, getteringmaterial/agent 231 can be deposited at the time that the reflectivelayer 233 is formed. In yet further embodiments, a gettering material231 is a dopant within the sacrificial layer 205 and 209 that isreleased during the etching of the sacrificial layers 205 and 209.

Now referring to FIG. 3 e, after surfaces of the cavities 221 and 223and/or the features 204 and 206 are treated and provided with a suitableenvironment, as described in detail below, the access trenches 213 and219 are preferably sealed. The release features 204 and 206 arepreferably sealed under a vacuum, but can be sealed within apredetermined or controlled gas and/or liquid for some applications. Theaccess trenches 213 and 219 are sealed by any of a number of methods andusing any of a number of materials including metals, polymers and/orresins. Preferably, the access trenches 213 and 219 are sealed bysputtering conventionally sputtered metals over the access trenches 213and 219 and the capping layer 211 and more preferably by sputteringaluminum over the access trenches 213 and 219 and capping layer to formthe layer 242.

Now referring to FIG. 3 f, for optical applications, a portion of thelayer 242 can be removed such that corking structures 240 and 241 remainin the access trenches 213 and 219. The capping layer 211 may provide anoptical window through which light can pass to the layer 233 on therelease features 204 and 206. Portions of the layer 242 are preferablyremoved by micro-polishing techniques. Alternatively, conventionalphoto-lithography techniques can be used to etch away a portion of layer242.

In an embodiment of the invention, portion of the layer 242 of the layeris selectively removed such that the capping layer 211 provides anoptical aperture (not shown) through which light can pass to and/or fromthe layer 233 on the release features 204 and 206.

FIG. 4 is a block diagram flow chart 300 outlining steps for forming amulti-layer structure shown in FIG. 3 a in accordance with a preferredmethod of the instant invention. The multi-layer structure shown in FIG.3 a is preferably made by sequential deposition processes, such asdescribed above, wherein the uniformity and thicknesses of each of thestructure layers are readily controlled.

Still referring to FIG. 4, in the step 301, a silicon dioxide layer isformed by steam or dry thermal growth on a silicon substrate or bydeposition on a selected region of the silicon wafer or other substrate.Preferably, the silicon dioxide layer is thermally grown to a thicknessin a range of 250 to 5000 Angstroms and more preferably in a range of250 to 750 Angstroms. The thermal oxidation occurs by placing the wafersubstrate at a temperature in a range of 600 to 800 degrees Celsius in acontrolled oxygen environment. In the step 303, a polysilicon layer ispreferably deposited by Low Pressure Chemical Vapor Deposition (LPCVD)on the first etch stop layer to a thickness in a range of 0.1 to 3.0microns and more preferably to a thickness in a range of 0.5 to 1.0microns. Low Pressure Chemical Vapor Deposition of the amorphouspolysilicon is preferably carried out at temperatures in a range of 450to 550 degrees Celcius.

After the first polysilicon layer is deposited in the step 303, then inthe step 305 a silicon nitride device layer is formed on the first polysilicon sacrificial layer. Preferably, the silicon nitride layer isformed by LPCVD to a thicknesses in a range of 300 to 5000 Angstroms andmore preferably in a range of 750 to 1250 Angstroms. The silicon nitridedevice layer can be formed by thermal decomposition of dichlorosilane inthe presence of ammonia.

In accordance with alternative embodiment of the current invention, thesilicon nitride layer is patterned with structure features after thedeposition of a photo-resist layer is deposited, exposed and developed(thereby forming an etch mask) in the step 303, or by selectivelyetching a pattern into the first polysilicon layer formed in the step303 to initiate rapid growth of the silicon nitride in the etched areasof the polysilicon layer. Preferably, the silicon nitride layer isdeposited as a continuous layer which is then selectively etched to formthe release features of the release structure using a conventionalphoto-resist mask.

After forming the patterned silicon nitride layer in the step 305, thenin the step 307 a second sacrificial layer is formed over the patternedsilicon nitride layer, sandwiching the patterned layer between the firstand the second sacrificial layers. The second sacrificial layer ispreferably also a polysilicon layer that is preferably deposited byLPCVD to a thickness in a range of 0.1 to 3.0 microns and morepreferably to a thickness in a range of 0.5 to 1.0 microns. The secondsacrificial layer is preferably formed by thermal decomposition of anorganosilicon reagent, as previously described. Preferably, the firstand the second polysilicon layer have contact points whereby the etchantcan pass through the contact points between the first and the secondsacrificial layers to etch away portions of both the first and thesecond polysilicon sacrificial layers. Preferably, in the step 311, andprior to the step 305 of forming the second polysilicon layer, thedeposition surface of the patterned silicon nitride layer is treatedwith a solvent such NMP (which can be heated) to clean its surface. Inaccordance with the method of the current invention, surfaces can betreated at any time during the formation of the multi-layer structure toremove residues thereon that may lead to poor quality films.

After the second polysilicon layer is formed in the step 307, then inthe step 309, a capping layer is formed over the second polysiliconlayer. The capping layer is preferably a silicon oxide capping layerdeposited by Plasma Enhanced Chemical Vapor deposition (PECVD) to athickness in a range or 1.0 to 3.0 microns and more preferably in arange of 1.5 to 2.0 microns. In the PECVD process, an organosiliconcompound, such as a tetraethyl orthosilicate (TEOS), is decomposed inthe presence of an oxygen source, such as molecular oxygen, to form thesilicon oxide capping layer. In the step 310, and prior to the step 309,the second polysilicon layer may be planarized and/or cleaned to preparea suitable deposition surface for depositing or forming the cappinglayer.

FIG. 5 is a block diagram flow chart 400 outlining the preferred methodof forming a device from the multi-layered structure shown in FIG. 3 a.In the step 401, access trenches are formed in the capping layer. Theaccess trenches are formed with diameters in a range of 0.4 to 1.5microns and more preferably in a range of 0.6 to 0.8 microns. The accesstrenches are preferably formed in the silicon oxide capping layer usinga reactive ion etch process. The reactive ion etch process can, underknown or empirically determined conditions, etch trenches with sloped orstraight walls which can be sealed in a subsequent step or steps. Theaccess trenches are preferably formed through the capping layer toexposed regions of the sacrificial material therebelow. Preferably, instep 402, and prior to the step 403, the exposed regions of thesacrificial layer are treated with a pre-etch cleaning solution ofethylene glycol and ammonium fluoride, that comprises approximately a10% by weight solution of ammonium fluoride dissolved in ethyleneglycol. After the exposed regions of the sacrificial layer are treatedwith the pre-etch solution in the step 402, then in the step 403 thepolysilicon layers are selectively etched with an etchant comprising anoble gas fluoride NgF_(2x), (wherein Ng=Xe, Kr or Ar, and where x=1, 2or 3). More preferably, the etchant comprises xenon difluoride. Furtheradvantages of using xenon difluoride etchant are described by Pister inU.S. Pat. No. 5,726,480, the contents of which are hereby incorporatedby reference.

After the etching step 403 is complete, then in the step 404 a getteringmaterial may be deposited through one or more of the access trenchesinto the device cavity formed during the etching step 403. In the step405, the access trenches are sealed by sputtering aluminum onto thecapping layer sufficiently to seal the access trenches. Excess aluminumcan be removed from the capping layer by well known methods such aschemical, mechanical polishing or phot-lithography.

FIG. 6 is a block diagram outlining the preferred method of etching thepolysilicon sacrificial layers in the step 403 shown in FIG. 5. Afterthe access trenches are formed in the step 401, and the exposed regionsof the polysilicon layer are treated in the step 402, as describedabove, then in the step 501, the structure is place under a vacuum ofapproximately 10⁻⁵ torr. In the step 503, xenon difluoride crystals arepreferably sublimed at a pressure in a range of 0.1 to 100 Torr, morepreferably in a range of 0.5 to 20 Torr and most preferably atapproximately 4.0 Torr. In the step 505, a controlled stream of xenondifluoride is provided to the chamber. The chamber is preferablymaintained at a pressure lower than the sublimation pressure of thexenon difluoride crystals to ensures a positive flow of the xenondifluoride to the chamber. The pressure in the chamber is preferablymaintained in a range of 0.1 milliTorr to 1.0 Torr, more preferably in arange of 1.0 milliTorr to 100 milliTorr and most preferably atapproximately 50 milliTorr (0.05 Torr).

FIG. 7 illustrates a schematic diagram of an apparatus 600 for carryingout the etching step described in block-flow diagram 500 shown in FIG.5. The apparatus 600 is preferably coupled with a vacuum source 607 thatis capable of drawing a vacuum in the chamber environment 605′. Theapparatus 600 preferably includes a pressure measuring device 609 thatallows a user to monitor the pressure within the chamber 610. Acontainer 608 containing an etchant source (e.g. crystals of xenondifluoride) is coupled to the chamber 610 through a pressure or flowcontroller 613. The container 608 can have a pressure measuring device611 coupled to the container 608 to allow the user to monitor thepressure within the container 608.

In operation, a multi-layer structure 620, similar to those describedpreviously, is placed in the chamber 610. The vacuum control valve isopened and the vacuum source 607 draws a vacuum reducing the pressure ofthe chamber environment 605′ preferably to or near to 10⁻⁵ Torr. Underknown conditions, the xenon difluoride crystals at room temperature forma vapor pressure of XeF₂ of approximately 4.0 Torr, as determined by thepressure measuring device 611. The pressure controller 613 is adjustedto change the pressure of the chamber environment 605′ to approximately50×10⁻³ Torr. The structure 620 is etched for a time sufficient to formthe release structure 623 within the cavity 621 of the structure 620.The etching process takes place over a period of approximately 20–30minutes, depending on the etching pressure chosen, the physical detailsof the structure 620 and flow dynamics of the chamber apparatus 600.

After the etching step is complete, a suitable sealing environment maythen be provided. Accordingly, in one embodiment the patrial pressurecontrol value 613 is shut off and a low pressure vacuum is reestablishedusing a draw from the vacuum source 607. The trenches of the etchedstructure 620 may be sealed by a sputter beam 650 of aluminum, using asputter device 630.

Alternatively, after reestablishing a low pressure vacuum, the chambermay be backfilled with a noble gas. Accordingly, a noble gas source 615may be coupled to the control chamber 610 through a control valve 612.The chamber environment 605′ is flushed with a noble gas by opening thegas valve 612 prior to sealing the trenches of the device 620. Thetrenches of the device 620 may then be sealed with a polymer or ceramicmaterial, thereby capturing a portion of the chamber environment 605′within the cavity 621 of the device 620.

The above examples have been described in detail to illustrate thepreferred embodiments of the instant invention. It will be clear to oneof ordinary skilled in the art that there are many variations to theinvention that are within the scope of the invention. For example, adevice with multiple layers of release structures can be formed byextending teachings of the invention and using multi-layer structureshaving more than one pattered layer. Further, it is clear that anynumber of devices with coupled and un-coupled release structures andwith multi-cavity structures are capable of being fabricated using themethod of the instant invention.

1. A release structure for fabricating a device, the structurecomprising: an etch stop layer resistant to etching by an etchantcomprising a noble gas fluoride; a sacrificial layer over the etch stoplayer, the sacrificial layer comprising a material to be selectivelyetched by the etchant; a capping layer over the sacrificial layer, thecapping layer serving as a passivation layer that packages the device;and an access opening through the capping layer, the access openingexposing the sacrificial layer to the etchant to allow etching of thesacrificial layer and release the device.
 2. The structure of claim 1wherein the noble gas fluoride comprises xenon difluoride.
 3. Thestructure of claim 1 wherein the capping layer is about 1.0 micron toabout 3.0 microns thick.
 4. The structure of claim 1 wherein thesacrificial layer is about 0.1 micron to about 3.0 microns thick.
 5. Thestructure of claim 1 wherein the etch stop layer comprises a materialselected from the group consisting of oxides, oxynitrides, and nitridesof silicon.
 6. A release structure for fabricating a device, thestructure comprising: an etch stop layer; a first sacrificial layer overthe etch stop layer; a patterned device layer over the first sacrificiallayer; a second sacrificial layer over the patterned device layer; acapping layer over the second sacrificial layer, the capping layerserving as a passivation layer that packages the device; and an accessopening through the capping layer, the access opening exposing thesecond sacrificial layer to allow etching of the second sacrificiallayer and the first sacrificial layer with a noble gas fluoride etchantto release the device.
 7. The structure of claim 6 wherein the noble gasfluoride etchant comprises xenon difluoride.
 8. The structure of claim 6wherein the capping layer is about 1.0 micron to about 3.0 micronsthick.
 9. The structure of claim 6 wherein the sacrificial layer isabout 0.1 micron to about 3.0 microns thick.
 10. The structure of claim6 wherein the first and second etch stop layers each comprises amaterial selected from the group consisting of oxides, oxynitrides, andnitrides of silicon.
 11. The structure of claim 6 wherein the first andsecond sacrificial layers each comprise polysilicon.